Radio receiving system



Feb. 24, 1931.

w. T. POWELL RADIO RECEIVING SYSTEM Fild May 29, 1924 INVENTOR Patented Feb. 24, 1931 UNITED STATES PATENT OFFICE WINFRED T. POWELL, OF ROCHESTER, NEW YORK, ASSIGNOR TO THE STROMBERG- GARLSON TELEPHONE MANUFAGTURENG COMPANY, OF ROCHESTER, NEW YORK, A

CORPORATION OF NEW YORK RADIO RECEIVING SYSTEM Application filed May 29, 1924 Serial No. 716,786.

This invention relates to systems for receiving high frequency radio signals and more particularly to a radio receiving system 1n which the so-called ,second harmonic heterodyne principle is used.

As is well known a system of this type operatesin general as follows: an incoming radio signal is caused to;beat with a locally generatedoscillation and the resultant wave is impressed 011 the input of a detector. The

detected signal in the output of the detector is then ampli ed by means of one or more intermediate frequency amplifiers after wh1ch M the signal is again detected and the resulting audio signal is then received on a telephone set either before or after passing through one or more audio frequency amplifiers. The local oscillator is operated at a fundamental frequency which is one half that required to give aresultant frequency of the proper value to operate the intermediate amplifier when the local oscillation beats with the incoming signal. The locally generated second harmonic is therefore of the proper frequency to heterodyne with the received signal for producing the properintermediate frequency.

Inasmuch as the fundamental frequency is one half the value of the second harmonic used when receiving a particular desired signal by this method, it is quite evidentthat an. undesired signal of the proper frequency can beat with the fundamental to produce the resultant intermediate frequency, and be detected at the same time as the desired signal.

This is objectionable and results in consider-y able interference.

An object of this invention is the provision of a circuit arrangement such that the second harmonic wave of a locallygenerated oscillation Wlll beat with an incoming signal while the first harmomcor fundamental wave a of the locally generated oscillation will be on the receiving antenna.

Further objects of the invention will appear in the following detailed description.

by curves how the arrangementof this invention serves to eliminate interference, due to undesiredslgnals beating with the 10- cally generated fundamental oscillations and also how radiation of the locally generated fundamental wave is prevented.

Referring to Fig. 1,. reference numeral 11 indicatesan antenna, which is connectedto ground through a tuned circuit consisting of variable condenser 12 and primary winding.

13 of coupler 19. Secondary coil 14 is bridged by variable condenser 15, these two latter elements comprising a, tuned secondarycircuit, which is connected to the input of the first detector tube in series with inductance coil 16. Coupled to inductance 16 areinductances 17 and 18 which are so wound and connected to the plates of the. oscillators that currents in these plate circuits, which are in phase will aid in their inductive effect on coil 16, while currents which are 180 degrees different in phase or in opposition will have no eflect on coil 16. y i

That part of the drawing within the dotted rectangle represents an oscillator unit con sisting of a primary oscillator which is coupled to a secondary oscillator 80, in such a manner that the former functions as, a driver for the latter. The output of oscillator 70 is coupled to the input of oscillator by means of condenser? 1 and variable resistance 72, the contact 73 being so adjusted that, as seen from tube 80, the effective amplification of tube 70 is unity.

Tube 80 functions to repeat in its output circuitthe wave appearing in the output of oscillator 70 and at the same time it produces a harmonic of its own. as hereafter described. Although the operation of tube 80 and its associated circuits resembles a combination of repeater, detector and amplifier,it will be referred to inthe description'and claims as an oscillator.

ihe tuned grid circuit type of oscillator is used in which variable capacity ll and inductance l2 constitute a closed oscillation circuit, the natural pe iod of which may be varied as desired. This oscillation circuit is connected to the grid 3 and filament l-l of vacuum tube 7 O by means of conductors 4-5 and 46 respectively. Adjustably coupled to inductance 4:2 is inductance l? which, in se ries with inductance 17, plate battery 48 and the plate-filament circuit within the tube, forms the output circuit of oscillator 70. The underlying principles of the operation of sucl an oscillator has been frequently described, one such description being given on page 8501 The Thermionic Vacuun 'lube by 'VanDeriBijl. I v

,Condense'r 71 and'resistance 72 are connected inseries across the plate-filament circuit of oscillator 70 and contact 73, which is variably connected to resistance-72, is connectedto grid 49, thus connectin the gridfilament circuit of oscillator 80 across a portion 'of'jthe output circuit of oscillator 70. This arrangementfunctions as a double oscillat-or the frequencies of the two being equal and the'amplitudes in the output circuits of both tubes being substantial ly the same. Inductance coilsl? and 18 in the output circuits of oscillators 70 and 80 are coupled to induc;

tance coil .16 andas above pointed out, are

sowoundand connected that currents flowingthroughcoils 17 and 18 which are 180 degrees'different in phase have substantially no eifect on coil 16.

Vacuum tube 50 is the first detector and the .detected beat or intermediate frequen wave appearing' in the output circuit of this tube is coupled by transformer54c to intermediate amplifier 51.- One or more such amplifiers may be used and the output of the last amplifier is coupled to the second detector 52 by means of transformer 55. The output circuit of detector is coupled to audio amplifier 53 by means of transformer 56. A C battery 57 is shown connected in the grid circuit of amplifier 53 and is for the purpose of maintaining the grid of this tube sulticient-ly negative for operation on the most desirable point of its characteristic curve.

58 and 59 indicate theusual grid condensers and grid leaks for detectors-52 and. 50.

It will be understood that additional audio amplificrs may be used for amplifying the audio signal to the proper amount for reception by the indicator 60 which may be a telephone or loud speaker. Condenser 61 acts as radio frequencyfby pass around battery 4'28 while 62, 63, 64, 65 and 66 are the usual filament rheostats. i 7 g I It will be assumed for the purpose of explanation that an incoming signal of a single irequency is i-mpressedacross the tuned secondary circuits 14, 15 andthat this frequency is 833 kilo-cycles. It willbe further assumed.

undesired incoming signal of 342 or 442 kilocycles isbeing impressed on the antenna, it

willbe seen that unless some means is provided to keep the fundamental from combining with this undesired signal they will interact to produce the intermediate or 50 kilocycle, oscillation and'the undesired signal will be received at the same time as the desired signal. I

The manner in which the locally generated fundamental frequency oscillation is kept out of the incomingor grid-filamentcircuit of the first detector, while the second ha monic oscillation is impressed on the incoming circuit will be readily understood by referring to l igs2and'3. v

On account of the curvature of the characteristic, the current wave in the platecircult of oscillator 'Z'O'is not a pure sinusoid, but is distorted as shownby curve 20 .ofjFig. 2. This curve-may be resolved into its two components, namely, the fundamental frek quency curve 21and the second harmonic curve 22. That thesecond harmonic wave 22. should be in the phase shown will be apparent by noting that it must be as shown in order that the algebraic sum of the fundamental 21 and the second harmonic 22 will at all points equal the resultant wave 20.

A certain amount of the current in the output of tube 70 is tapped ofl' and impressed upon the grid of tube 80 and since the potential impressed on grid 49 is of such value that the current Wave in the output of tube 80 is of the same amplitude as that in the output of tube 70, this current wave will be as shown' by curve 30 of Fig. 3. This wave will also be distorted because of the curvature ofthe characteristic ,oftube 80. Since oscillator 80 is driven by oscillator 70, the frequency of wave 30 will be the same as wave 20; but it will be 180 degrees out of phase with wave 20, due to the fact that an increase of current in the output of tube 70 produces a decrease of current in the output of tube 80.

Curve 30 may be resolved into its funda mental curve 31 and second harmonic curve 32. That curve 32 is in the phase shown. is for the same reason as pointed out above for tube/ 70. It should be noted that curves 22 put oftube 7 O, a certain amount of it will be impressed on the input of tube 80 and will be reproduced in the output of this tube, but will be'180 degrees out of phase with curve 22 as shown by curve 33. The reason curve 33 is 180 degrees out of phase with curve 22 is also due tothe factthat an increase of current in the outputof tube produces a decrease of current in the output of tube 80.

Since fundamental curves 21 and 31 are 180 degrees out of phase'and as these curves represent currents flowing through coils 17 and 18 respectively, it will be seen, by noting the current directions through these coils, as shown by the arrows, that the fundamentals will neutralize each other and will not induce currents in coil 16. It will also be noted that the second harmonic waves represented by curves 22 and 33 are also 180 degrees out of phase and will neutralize each other with the result that they will not induce currents in coil 16. Curve 32, however, which represents a second harmonic wave, is not cancelled and it will be effective to induce a second har- 1 monic current intocoil 16,.which will beat with the incoming signal impressed on this coil by tuned circuits 14, 15 and the resultant wave will then be impressed on the gridfilament circuit of the detector, detected and amplified in the welf known manner.

Since thefundamental frequency of the oscillator is not fed into coil 16, it will be apparent that thisfrequency, which 1s of considerable amplitude, will not be radiated from the antenna. It therefore, follows that this invention provides a means for substantially eliminating the fundamental frequencies developed by the amplification of alternating current waves by devices having a curved characteristic.

This invention, however, is not limited to the case where the two stages are exactly similar. Even where the tubes have widely different characteristics the fundamentals may be considerably reduced by the adjust ment of potentiometer? 2, so that the funda mentals in the output of tube 70 will be sub stantially neutralized by the fundamentals in the output of tube 80.

It is to be understood that the circuit arrangement above described may be variously modified without departing in any way from the spirit of this invention, as defined in the appended claims.

What I claim is:

1. The combination in a signal receiving system of an incoming circuit upon which signals are impressed, a local oscillation circuit for impressing first and second harmonic ourrent waves upon said incoming circuit, and means for neutralizing said first harmonic current waves.

2. The combination in a signal receiving system of an incoming circuit upon which signals are impressed, an oscillation circuit for impressing fundamental and harmonic ourwave of two components, one of which is a a harmonic component, and means for neutralizing one of said components.

1. The combination in a signal receiving system of an incoming circuit upon which a signal wave is impressed, an oscillation circuit upon which a local wave consisting of two components is impressed, one of said components being a harmonic component, a detector for receiving a resultant wave and means for combining said signal wave with one co1nponent only of said local wave to produce said resultant wave.

5. The combination in a signal receiving system of an incoming circuit for receiving a high frequency signaling current, an oscillator for producing a plurality of high frequency localcurrents, and means including a detector network for combmln said hi h he a v I b b quency signaling current with one of sald hi 'h ire uenc local currents to the exclusion p of the other of said high frequency local currents.

6. A stage in the method of receiving an alternating current signal wave, which comprises the step of impressing said wave upon a device having an input voltage-output current curved characteristic, whereby a fundamental and harmonic waves of said alternating current wave are produced, impressing said fundamental and said harmonic waves upon a second device having an input voltageoutput current curved characteristic whereby the phase of the fundamental and certain of the harmonic waves are changed, and combining said waves to substantially eliminate said fundamental and certain of said harmonic waves.

7 The method of detecting an alternating current signal wave which comprises the steps of producing a fundamental and a harmonic frequency, developing from said fundamental frequency a phase displaced fundamental frequency, combining said fundamental frequencies whereby they are neutralized, and changing the frequency of the remaining harmonic frequency by combining it with said signal wave.

In witness whereof, I hereunto subscribe my name this 27th day of May, A. D. 1924.

WINFRED T. POW'ELL. 

